Investment Securities (Narrative) (Detail) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2016 USD ($) securities	Sep. 30, 2015 USD ($)
Available-for-sale Securities and Held-to-maturity Securities [Abstract]
Available-for-sale securities, transferred to held-to-maturity securities	$ 862.7	$ 0.0
Available-for-sale securities, transferred to held-to-maturity, unrealized gain (loss)	14.4
Pledged Securities, carrying value	$ 1,200.0	$ 1,400.0	$ 1,300.0
Number of securities by a single non-goverment sponsored issuer exceeding 10% of shareholders' equity | securities		0